Hime Musha

Main character is a student who has to babysit three sisters after parents' death. The mysterious beautiful girl trio called "Hime-hime" becomes the talk of the world as they are rumored to prevent the assassinations of government officials every time.
But main character himself becomes that target of attacks of a girl in battle suit. He can't hide his surprise as his sister transforms into Himehime to protect him.

Protagonist appears to come from a powerful clan himself and adopted by Fujiwara family, thus he requires protection as an important person of power. Pressed between lovely sisters and enthralling transfer student, who will he choose?

[From vndbreview]